The present invention relates to a memory apparatus such as a magnetic disk apparatus or the like using an MR head as a read head and, more particularly, to a memory apparatus for correcting an asymmetry of a read waveform of the MR head in association with an offset correction.
In recent years, in the magnetic disk apparatus used as an external memory apparatus, the improvement of a higher recording density, a higher transfer speed, and a higher spindle rotational speed have been realized. Use of a MR head (Magnet-Resistive Head) which can secure a high output level without depending on a rotational speed (circumferential speed) of a disk medium has begun. However, an upper/lower asymmetry which is peculiar to a read waveform exists in the MR head. If there is an upper/lower asymmetry, an error rate upon reading is deteriorated. Such an upper/lower asymmetry of the MR head can be eliminated by adjusting a bias current (sense current) to be supplied to an MR device provided for the MR head.
FIG. 1 shows a curve 112 of .rho.-H characteristics (resistivity--magnetic field characteristics) of the MR device. Different operation points 114, 120, and 126 are determined for the curve 112 in accordance with a magnetic field (H) from a disk. Output waveforms 118, 124, and 130 from the MR device are obtained in accordance with changes 116, 122, and 128 in the recording magnetic field (H) at the operation points 114, 120, and 126. In this instance, since the operation point 114 is located in a straight line area in which an inclination of the curve 112 is almost constant, the output waveform 118 which is symmetrical with respect to the upper and lower portions for the magnetic field change 116 is obtained. On the other hand, since the operation point 120 is located in an area in which the inclination of the curve 112 decreases in accordance with a decrease in magnetic field (H), the asymmetric output waveform 124 in which an amplitude is small on the upper side and is large on the lower side is obtained for the magnetic field change 122. Since the operation point 126 is located in an area in which the inclination of the curve 112 increases in accordance with the decrease in magnetic field (H), the asymmetric output waveform 130 in which an amplitude is large on the upper side and is small on the lower side for the magnetic field change 128 is obtained. In this instance, assuming that an amplitude on the upper side of each of the output waveforms 118, 124, 130 is set to A1 and the amplitude on the lower side is set to A2, the upper/lower asymmetry Asym. of the output waveform is defined by the following equation. EQU Asym.=(A1-A2)/(A1+A2).times.100[%] (1)
In FIG. 1, since A1=A2 at the operation point 114, the upper/lower asymmetry Asym.=0% and the asymmetry is eliminated. However, since A1&lt;A2 at the operation point 120 and A1&gt;A2 at the operation point 126, an asymmetry occurs. In order to eliminate such upper/lower asymmetry of the read waveform which is peculiar to the MR head, it is necessary to optimize a bias current flowing into the MR device and to set the operation point of the magnetic field to (H) a proper position. In the magnetic disk apparatus using a conventional MR head, the bias current to be supplied to the MR device in the MR head is adjusted, at a final assembling stage so that the asymmetry Asym. is equal to 0% and the current becomes symmetrical with respect to the upper and lower portions in a state in which the MR head is on-track controlled. A set value of the bias current at which such a read waveform without an upper/lower asymmetry can be obtained is stored in a memory of the apparatus every head. Therefore, at the time of the reading operation when the apparatus is used, the set data of the bias current is read out from the memory in accordance with the head number and the optimized bias current is supplied to the MR head, thereby preventing that the upper/lower asymmetry of the read waveform occurs. As for the adjustment of the bias current, for example, in a state in which the magnetic disk apparatus is connected to a testing machine and the MR head is on-track controlled to a predetermined cylinder position, the asymmetry Asym. according to the equation (1) is measured from the output waveform while adjusting the bias current to be supplied to the MR head. The bias current whose measurement value is equal to 0% is decided and current set data for an A/D converter used for supplying such a bias current is stored in the memory of the apparatus.
According to a servo surface servo method of the magnetic disk apparatus, for example, servo information recorded on a servo surface is read out by a servo head, a combination head integratedly having a write head and a read head is controlled so as to be located at a target cylinder position, and a thermal positional deviation depending on a use temperature or the like of the apparatus exists between tracks at the same cylinder position of the servo surface and data surface. An offset of the head for the track center due to the positional deviation is corrected. That is, at the time of a diagnosing process when the apparatus is power-on-started, the positional deviation is measured and stored into the memory of the apparatus every head number and every cylinder number. At the time of the subsequent on-track control in the reading or writing operation, an offset amount is read out from the memory, and the offset of the combination head is corrected. In the case where the apparatus is used for a long time, further, since an amount of thermal offset which occurs for such a period of time is changed, a calibration to measure the offset is performed at predetermined time intervals, thereby updating the offset amount of the memory.
According to the conventional magnetic disk apparatus using such an MR head, however, although the bias current for eliminating the upper/lower asymmetry of the MR head is measured in a state in which the MR head was on-track controlled, an offset correction of the thermal offset or the like is not executed at this time. At some later point in time, during a reading operation, the offset correction to eliminate the thermal offset is executed by using a result of the measurement of the thermal offset in association with an initial diagnosing process. Therefore, the on-track position of the MR head when the thermal offset is corrected by the reading operation differs from the on-track position used when determining the bias current in accordance with the offset correction amount. As mentioned above, when the MR head is on-track controlled to a position different from the position at which the bias current was decided by the offset correction, the operation point at the time of the actual reading operation is changed for the adjusted operation point 116 in FIG. 1 and the upper/lower asymmetry occurs in the read waveform. When the upper/lower asymmetry appears in the read waveform, so long as a peak detecting method is used in a demodulating circuit of the read waveform, in case of a level detection of a differential waveform after completion of a full-wave rectification, it causes a loss of margin for a slice level and a possibility such that an error rate rises. In the case where a PR4ML (Partial Response Class 4 Maximum Likelihood) method as a level detecting method is used in the demodulating circuit, an equalization error by an equalization occurs because sampling points of the upper and lower waveforms are deviated, so that it similarly becomes a factor to affecting the error rate.